The present invention relates generally to induction machines, and, more particularly to a system to improve the torque smoothness of an electric machine.
In the automotive industry, package space is budgeted for under hood and engine components. Starters and alternators with toroidal windings offer a space savings over stators with conventional lap windings. Lap windings require end space that increases the length of the machine. End space is needed to route a coil from one stator slot to another stator slot that may be several slots away according to the pitch or spacing of the coils.
The end turn, the length of the coil that extends from between the slots from coil side to the other coil side, extends beyond the stator laminations. This increases the length of the stator by the distance required for the coil to make the turn from one slot for one side of the coil to the corresponding slot for the other side of the coil. Toroidal windings conserve space between the large turning area for end turns is not required. Toroidal coils are wound about the core with one side of the core laid in the slot on the interior of the laminations and the other side of the coil being laid on the outer periphery of the laminations. However, even with toroidal windings, there are end turns that extend beyond the laminations because of the space required for the coil to make the turn to go from one side of the laminations to the other side of the laminations.
In a vehicle, the alternator is subjected to constant operation generating heat that must be dissipated. Conventional toroidal windings have coils surrounding the core or the stator compounding the problem of heat dissipation because the core or stator cannot be used to dissipate coil heat, as is the case with lap windings. Hot spots, therefore, exist where the coil is not in uniform contact with the core laminations. Further, toroidal coils are layered placing one layer closer to a heat sink so that the heat is not transferred uniformly. Another problem with some toroidal windings is that the inner diameter of the core is smaller than the outer diameter so that spacers must be hand positioned when assembling the coils to prevent coil movement. Adding spacers during assembly complicates the assembly process and slows production. It is, therefore, desirable to have a stator with toroidal coils that have minimal end turn and promote heat dissipation.
To solve these problems, typically, alternators and starters with high phase counts are used to minimize toroidal windings. A problem with high phase order AC drive systems is the necessity of measuring phase currents so that the electric machine phase currents can be regulated. For example, in the specific case of a 9-phase induction machine, this means eight current sensors. Because of the complexity of using eight current sensors, the automotive industry has found alternatives allowing them to use only two current sensors.
Compared to 3 phase-4 pole configuration, the 9 phase-4 pole configuration increases torque smoothness. However, due to the complexities in implementation, in addition to current sensor costs, 9-phase control is not practical. Instead, conventional three-phase control is employed. Only two currents sensors are needed and control strategies of three phase systems are well defined. However, in this induction machine, going from nine to three phase control results in MMF wave resolution reduction. The quality of the MMF waves directly effects the torque smoothness. Therefore, there is a desire to maintain 9-phase quality MMF resolution, while using conventional three-phase control.
It is, therefore, an object of the invention to reduce weight and improve reliability of an automotive starter/alternator. Another object of the invention is to improve the torque smoothness of an automotive starter/alternator.
In one aspect of the invention, a stator for an induction machine comprises a cylindrical core that has a plurality of inner and outer slots and a plurality of toroidal coils wound about the core and laid in the inner and outer slots. Each toroidal coil occupies a single inner slot and is laid in the adjacent associated outer slot to minimize the distance the coils extend from the end faces of the core thereby minimizing the length of the induction machine. The coils are encapsulated in a material that fills any voids to provide a thermal path for uniform heat dissipation.
The area of the outer slots is equal to the area of the inner slots. When the inner slots are full, the coils fit exactly in the outer slots. An exact fit increases mechanical integrity of the structure, and a single layer winding minimizes end turn build up while providing a thermal path.
The present invention enables arbitrary combination of the number of poles and phases. Its principle of operation will be illustrated on an example of a 4-pole, 9-phase alternator and a 12-pole, 3-phase motor.
In operation, a controller is coupled to three current sensors and detects the current of only three phases. These phases are then used to calculate the current of the adjacent six phases, thereby, allowing for better control of the electric machine.
The present invention thus achieves a system for improving the torque smoothness of an electric machine. This allows lower weight and higher reliability in automotive applications. Additional advantages and features of the present invention will be come apparent from the description that follows and may be realized by the means of the instrumentalities and combinations particularly pointed out in the appended claims taken in conjunction with the accompanying drawings.